Open trailers with tail gates are often used to transport lawn care equipment and other products. As the tail gate may be long and heavy, it can be difficult to operate by hand. In a typical arrangement, with a tail gate that is five feet long from the hinge to the free end, a force of 80 pounds must be applied at the free end of the tail gate in order to lift the tail gate when it is in the open position. Several types of lift devices have been used in the past, but they all have problems. In many cases, the lift device includes a spring or other elastic member which substantially increases the resistance against which a person must work in order to open the tail gate from its closed position, making it much more difficult to open the tail gate than if there were no assist at all. So, while the elastic member does provide an advantage in that it helps with lifting the tail gate, it also creates a substantial disadvantage by making the tail gate more difficult to open than it would be without the assist.
For example, U.S. Pat. No. 6,485,004 “Licata” shows a lift spring for a tail gate in which the lift spring is mounted to the side rail of the trailer and to the tail gate. The spring extends upwardly from the side rail to the tail gate when the tail gate is closed and downwardly from the side rail to the tail gate when the tail gate is fully open. The spring is in tension throughout the entire distance of travel of the tail gate and its length does not increase substantially from the raised position to the lowered position, so the spring force does not change appreciably from the closed position to the open position. In this case, since the spring force is roughly the same throughout the travel of the tail gate and since the angles at which the spring applies its force are not advantageous, the operator must exert a substantial force against the spring in order to open the tail gate, and very little of the spring force actually helps counteract the weight of the tail gate in order to help the operator raise the tail gate. At the beginning of travel from the closed position, the spring is pulling primarily downwardly on the tail gate, but it also exerts a horizontal force against which the operator must pull in order to open the tail gate. As the operator begins to open the tail gate and throughout the rest of the travel of the tail gate, the spring force acts primarily in the horizontal direction, so the operator has to pull the tail gate outwardly against that spring force in order to open the tail gate. Even in the fully open position, the spring angle is such that most of the spring force is acting in the horizontal direction and very little of the spring force is acting in an upward direction to help counteract the weight of the tail gate.
U.S. Pat. No. 6,126,223 “Rayburn” mounts an elongated assist system along the top of the side rail of the trailer. This occupies a substantial distance along the side rail, which is undesirable, because it prevents that space from being used for other purposes. In this design, the elastic member includes a spring and a cable mounted onto the spring. The cable passes over rollers and the elastic member does not remain in a straight line but rather bends around the rollers. Again, the elastic member does not elongate appreciably from the fully closed position to the fully open position, so it is exerting a substantial spring force against the tail gate in all positions. In order to begin opening the tail gate, the operator must apply a substantial horizontal force to counteract the horizontal force of the spring, and most of the spring force continues to be applied in a horizontal direction, even at the fully opened position of the tail gate, so only a small portion of the spring force actually helps act against gravity to help lift the tail gate.